[PATCH net v2 1/2] tls: don't leave a full plaintext sk_msg ring unpushed

[chanyoung <ppoo1220@gmail.com>]

When the copy path in tls_sw_sendmsg_locked() adds the fragment that fills
the plaintext sk_msg ring, it does not set full_record, so the record is
left full and unpushed.  A later splice() then adds to an already full
ring: sk_msg_page_add() has no fullness check of its own, so sg.end wraps
onto sg.start and the ring appears empty.  Fragments added after that
overwrite live entries, and sg.size no longer matches what is reachable
between sg.start and sg.end, so pushing the record runs the scatterwalk off
the end of the scatterlist.

An unprivileged user can trigger this on a loopback TCP socket with the
"tls" ULP attached:

  BUG: kernel NULL pointer dereference, address: 0000000000000008
  RIP: 0010:memcpy_from_scatterwalk+0x32/0xc0
  Call Trace:
   skcipher_walk_next+0x1d1/0x2c0
   gcm_encrypt_aesni_avx+0x1e9/0x220
   bpf_exec_tx_verdict+0x3bb/0x860
   tls_sw_sendmsg+0xa1a/0xca0
   __sys_sendto+0x1da/0x1f0

Set full_record in the copy path when the ring becomes full, and push a
record that is already full on entry to the sendmsg loop.

Suggested-by: Sabrina Dubroca <sd@queasysnail.net>
Fixes: fe1e81d4f73b ("tls/sw: Support MSG_SPLICE_PAGES")
Cc: stable@vger.kernel.org
Signed-off-by: chanyoung <ppoo1220@gmail.com>
---
 net/tls/tls_sw.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index d4afc90fd79..d2e399be8ef 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -832,6 +832,14 @@ static int tls_sw_sendmsg_locked(struct sock *sk, struct msghdr *msg,
 		if (!sk_stream_memory_free(sk))
 			goto wait_for_sndbuf;
 
+		/* open record may be full if we couldn't push it in the last sendmsg call */
+		if (sk_msg_full(msg_pl)) {
+			full_record = true;
+			sk_msg_trim(sk, msg_en,
+				    msg_pl->sg.size + prot->overhead_size);
+			goto copied;
+		}
+
 alloc_encrypted:
 		ret = tls_alloc_encrypted_msg(sk, required_size);
 		if (ret) {
@@ -921,6 +929,12 @@ static int tls_sw_sendmsg_locked(struct sock *sk, struct msghdr *msg,
 						       msg_pl, try_to_copy);
 			if (ret < 0)
 				goto trim_sgl;
+
+			if (sk_msg_full(msg_pl)) {
+				full_record = true;
+				sk_msg_trim(sk, msg_en,
+					    msg_pl->sg.size + prot->overhead_size);
+			}
 		}
 
 		/* Open records defined only if successfully copied, otherwise
-- 
2.43.0